This invention relates to high-purity trimethylaluminum, and also to a purification method of crude trimethylaluminum.
Compound semiconductor materials have uses in the electronics industry in such applications as microwave oscillators, semiconductor light-emitting diodes, lasers and infrared detectors. The quality of a compound semiconductor available from an epitaxial growth of an organometallic compound is significantly controlled by impurities, especially those in the organometallic compound as a raw material. Therefore, high purity is required for the organometallic compound to obtain the compound semiconductor material with a high function.
The impurities in an organometallic compound, especially in trimethylaluminum, include hydrocarbon components, organosilicon components, alkylaluminum oxides, metal compounds, etc. Among these impurities, organosilicon components and alkylaluminum oxides generally have higher or similar vapor pressures compared with trimethylaluminum so that they may form silicon inclusions and oxygen inclusions in a compound semiconductor to be produced from trimethylaluminum, and therefore, are considered to be particularly harmful.
Further, the metal compounds are also considered to be compounds having higher or similar vapor pressures compared with trimethylaluminum, such as titanium compounds, zinc compounds, and sulfur compounds.
The alkylaluminum oxides, on the other hand, are considered to be included or formed during the synthesis and handling operations of trimethylaluminum, and are regarded as one of factors that deteriorate the qualities of compound semiconductors.
As a conventional purification process of crude trimethylaluminum, the distillation process is widely known. With simple batchwise distillation or continuous distillation, however, it is difficult to remove organosilicon components or alkylaluminum oxides, because the inclusion of such impurities are considered to be attributable to the production process, production facilities and process operations of crude trimethylaluminum as a raw material.
Known production processes of trimethylaluminum may be divided roughly into the following three types of processes:    (1) A methylaluminum chloride synthesized from aluminum and methylene chloride is brought into contact with sodium or magnesium, followed by reduction to afford trimethylaluminum (sesquichloride process).    (2) Aluminum is activated with an alkylaluminum and hydrogen. Ethylene is reacted to the resulting activated aluminum to obtain triethylaluminum, and by a substitution reaction, trimethylaluminum is then afforded (direct process).    (3) A mixture of aluminum, isobutylene and hydrogen is activated and reacted with a catalyst to obtain triisobutylaluminum. By a substitution reaction with ethylene, triethylaluminum is obtained, followed by a substitution reaction with methyl chloride to afford trimethylaluminum (isobutylene substitution process)
As the above processes use aluminum and an alkali metal or aluminum, it is considered that impurities such as silicon, iron, zinc, magnesium and sulfur are readily included in trimethylaluminum.
These impurities are considered to change into various compounds in the production steps. In particular, silicon, zinc and sulfur are considered to change into substances that readily include in organometallic compounds having one or more methyl groups, for example, trimethylaluminum and organometallic compounds available from trimethylaluminum as a raw material such as trimethylgallium and trimethylindium.
More specifically, in the case of organosilicon compounds, they include compounds represented by Si(CH3)x(C2H5)yCl4−x−y and Si(OCH3)x(C2H5)yCl4−x−y (x=0 to 4, y=0 to 4) such as tetramethylsilane, trimethylmethoxysilane, trimethylchlorosilane, silicon tetrachloride, and triethylmethylsilane. Alkylaluminum oxides and metal compounds are considered to include (CH3)2AlOCH3 and (CH3)Al(OH), and Zn(CH3)2, SCl2 and S2Cl2, although they are not limited thereto.
Further, the contents of the impurities are in the order of ppm or ppb. Such extremely low contents have made it difficult to establish their removal processes.
For the high purification of such crude trimethylaluminum, there have heretofore been reported adduct purification processes (JP-B 5-35154), processes involving distillation in contact with metallic sodium or metallic potassium (JP-A 62-132888), processes for purifying liquid organometallic compounds by cooling and solidifying them (JP-A 8-12678), and processes involving mixing of an organometallic compound containing one or more halogen and/or hydrogen atoms with an alkali halide and a subsequent heat treatment (JP-A 7-224071).
However, the adduct purification processes are accompanied by many drawbacks in that, as they require the addition of a solvent and the chemical for the treatment to trimethylaluminum to be purifyed, the purity of the solvent and chemical need to be made very high before its addition, the recovery rate of trimethylaluminum is low, the solvent and chemical have to be treated after use, the chemical is costly, and the operation is cumbersome.
The processes involving distillation in contact with metallic sodium or metallic potassium can separate organosilicon components to some extent, but the removal rate of organosilicon components is insufficient for compound semiconductors, especially for applications which require high purity.
The processes involving the cooling and solidification of organometallic compounds also have problems in that the removal rate of organosilicon components or alkylaluminum oxides from trimethylaluminum is not only unstable but also insufficient to obtain satisfactory results, require complex facilities for industrial practice, and also require cumbersome operations.
The processes involving mixing of an organometallic compound containing one or more halogen and/or hydrogen atoms with an alkali halide and a subsequent heat treatment to remove alkylaluminum oxides are also accompanied by problems in that their effect is limited to alkylaluminum oxides and moreover, their removal effect is insufficient.